<p>This study employed hydrogen-oxygen stable isotopes (δ¹⁸O, δD) and tritium isotopes (³H) as tracers, combined with the MixSIAR model, to conduct a quantitative analysis of recharge sources in the Maocun Karst underground river basin. Two hundred thirty-two samples of surface water, spring water, and groundwater were collected during the continuous hydrological year from September 2020 to August 2021, revealing the conversion relationships and recharge mechanisms among different water bodies in the basin. The results show that: (1) The Maocun underground river is mainly replenished by atmospheric precipitation, with δ¹⁸O and δD exhibiting a “lighter in wet periods, heavier in dry periods” characteristic, indicating significant evaporation and more vigorous evaporation intensity during the rainy season; (2) The MixSIAR model quantitatively shows that the recharge of upstream spring water is approximately 45% from external water sources (Xiaolongbei and Modaojiang surface rivers) and the dolomite aquifer. The downstream underground river outlets (Dayanqian and Maocun) are primarily replenished by upstream spring water (46.5%) and the limestone aquifer (more than 39%); (3) ³H isotope analysis indicates that groundwater in the basin mainly consists of a mixture of pre-1952 and recent waters (86%-90%), with only a tiny proportion (9%-14%) being modern water (rainwater). The water cycle alternates more actively in wet periods. This study quantitatively analyzed the multi-source recharge sources of typical karst groundwater, providing scientific support for the sustainable development and protection of groundwater resources in karst areas.</p>